Use of gas recirculation for superheat temperature control



July 6, 1965 E. RICHARDSON USE OF GAS RECIRCULA'I'ION FOR SUPERHEAT TEMPERATURE CONTROL Filed D60. 27, 1963 2 Sheets-Sheet 1 FIG. I

INVENTOR ELVA RICHARDSON W x 014w July 6, 1955 E. RICHARDSON USE OF GAS RECIRGULATION FOR SUPERHEAT TEMPERATURE CONTROL 2 Sheets-Sheet 2 Filed Dec. 2'7, 19.65

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INVENTOR. ELVA RICHARDSON United States Patent O" 3,192,909 USE OF GAS RECIRCULATION FOR SUPERHEAT TEMPERATURE CONTROL Elva Richardson, West Hartford, Conn., assignor to Combastion Engineering, Inc, Windsor, Coun., a corporation of Delaware Filed Dec. 27, 1963, Ser. No. 333,903 1 Claim. ((21. 122-478) This invention relates to steam temperature control in a steam generator, and in particular to apparatus for effectively maintaining constant superheat temperature while the unit is operating over a wide load range by utilizing gas recirculation.

In most steam generators, the steam generating surface and steam superheating surface is so proportioned that the unit will operate most efficiently at full load, while producing superheated steam at a given temperature. It is necessary to maintain the superheat temperature constant regardless of 'what load the unit is operating at, and this presents problems since the superheat temperature in most units tends to decrease with decrease in load. One solution to this problem is to recirculate cooled combustion gases from the rear gas pass back to the furnace, and to increase the amount of such gases recirculated back to the furnace as the load on the unit decreases. This decreases the amount of heat transferred to the radiant steam generating surface, lining the furnace walls while increasing the amount of heat transferred to the convection superheat surfaces located outside of the furnace, thus holding the superheat temperature substantially constant even though the load on the unit decreases.

It is an object of this invention to provide a gas recirculation arrangement which allows constant superheat temperature in an effective manner, regardless of what load the unit is operating at.

Other objects and advantages of the invention will become apparent from the following description taken in conjunction with the accompanying drawings wherein:

FIGURE 1 is a'vertical section of a steam generator incorporating my novel gas recirculation arrangement; and

FIGURE 2 is an enlarged view of the lower portion of the front furnace Wall.

In FIGURE 1 numeral designates a steam generating unit having a furnace 11. Fuel is supplied to a plurality of short-flame burners :12 by means of fuel supply line '14. Air to support combustion of the fuel is supplied to the burners by way of air duct 16 from forced draft fan 20. The air is heated in rotary regenerative air heater 18 prior to its introduction into the furnace.

Water is introduced into the economizer heat exchangers 22 through inlet line 24. The heated water flows from economizers 22 through tubes 26 to the steam and water drum 28. The water then flows through downcomer 20 to distributing header '32, thence to a plurality of smaller headers, one of which is shown at 33 in FIGURE 1. The water leaving header 33 flows through steam generating tubes 34, which line the front furnace wall, and also through steam generating tubes 35 which line the furnace floor, and are then bent upwardly to also line the rear furnace wall.

As shown in FIGURE 2, the side walls of the furnace are also lined with steam generating tubes 70 and 74, which tubes are supplied from headers 72 and 76, respectively.

The steam and water mixture leaving the furnace tubes is discharged into drum 28, where the steam is separated and flows through tubes 38 to the primary superheater 36. The steam then flows to final superheater 40 by means of connecting tubes 42, and the superheated steam leaving section 49 passes through outlet line 44 to its ultimate point of use, for example a high pressure steam 3,1919%? Patented July 6, 1965 the turbine is then returned to the boiler and passes through reheater 46 and then returns to an intermediate pressure turbine.

The hot combustion gases created by the burning of fuel within furnace 11 flows upwardly through the furnace, and then enters the gas passageway 48 and flows downwardly therethrough, passing through airpreheater 18 prior to its discharge to the atmosphere by way of stack 50.

As hereinbefore mentioned, the steam generating surface and steam superheating surface are proportioned such that the unit operates most efficiently at full load, while maintaining a predetermined temperature of the superheated steam leaving the unit through outlet line 44. When the unit is operated at less than full load, the steam generating tubes 34, 35, 7t and 74 which line the floor and walls of the furnace absorb a larger proportion of the total heat released, while the superheater surface, 40 absorbs a lesser proportion. .Thus unless some type of control is provided, the superheat temperature falls oif with a decrease in load.

To overcome this decreasing superheat temperature, a duct 52 is provided which extends from the gas passageway 48 to the furnace 11. Relatively cooled combustion gases are thus recirculated from the gas passageway 48 to the furnace 1.1 by means of fan 56 when damper 58 is in a position other than fully closed. These recirculated combustion gases are relatively cool compared to the com bustion gases released by the burning of the fuel within the furnace, since a major portion of their heat has been given up to the steam generating tubes lining the furnace walls, and also to the convection heat exchange surfaces contained within the gas passageway 48. These recircu lated gases thus result in a decrease of the over-all furnace gas temperature, and also result in a greater velocity of the gases flowing out of the furnace, due to the increased volume. This results in less heat being given up to the steam generating tubes lining the furnace walls, thus increasing the heat available in the gases which pass over the superheater surface 40.

The amount of gases recirculated can be controlled by any suitable means. For example, temperature sensing means .60 positioned in line '44 can be used to send a signal to operating means for damper 58, which will open the damper more as the temperature decreases in superheat outlet line 44.

The introduction of recirculated gases through restricted openings 54 positioned belowthe lowermost short-flame burner 12 results in a very effective superheater temperature control arrangement. As shown in FIGURE 1, the recirculated gases form an effective blanket covering the entire furnace floor, which also extends upwardly along the rear furnace wall, thus effectively blanketing a large portion of the steam generating surface. In' thismanner, the blanket of recirculated gases has its greatest depthdirectly under the area within the furnace where the greatest amount of burning takes place, and thus also where the greatest combustion gas temperature exists. This blanket of cooled combustion gases thus prevents to a large extent heat transfer from the furnace combustion gases to the furnace floor tubes 35 by radiation; and also prevents heat transfer to the tubes 35 lining the rear furnace wall. As far as the invention is concerned, it makes no difference whether the tubes lining the furnace walls are tangent, or are of finned construction. As shown in FIGURE 2, the tubes lining the front furnace Wall are of finned construction, while the tubes 35 which line the furnace floor and also extend upwardly to form the rear wall, are tangent tubes with no fins therebetween. The restricted openings 54 which allows the recirculated gases to enter the furnace =11 are formed by omitting portions of the fins.

As shown in FIGURE 1, the corner joining the floor tubes and the rear wall tubes is rounded. This prevents undue turbulence of the recirculated gases as it flows around the corner, and thus helps to a large extent in maintaining an effective blanket of cooled gases flowing upwardly along the rear wall 35.

While I have shown and described the preferred embodiment of the invention, it is to be understood that the invention is not limited thereto, but may be otherwise variously embodied and practiced within the scope of the following claim.

What I claim is:

In a steam generating unit, a furnace having front, rear, side and bottom enclosing wall means, said bottom Wall means being substantially horizontal, all of said wall means including steam generating tubes, short flame burner means mounted in the lower portion of the front wall means, gas passageway means connected to the upper portion of the rear wall means, steam heating means positioned within the gas passageway means, outlet means positioned in the front wall means below the short flame burner means, said outlet means extending substantially across the-entire front wall means, a duct having an inlet end connected to the gas passageway means downstream of the steam heating means, and having an outlet end flame burner means and the bottom wall means, the cor-' ner joining the bottom wall means and the rear wall means being rounded sufiiciently to prevent undue turbulence of the cooled combustion gases as they flow around such corner, so that an effective blanket of cooled combustion gases is also formed between the flame and the rear wall means.

References Cited by the Examiner UNITED STATES PATENTS 2,876,748 3/59 Nelken 122-478 2,905,156 9/59 Pecault 122479 2,976,858 3/61 Chan 122479 3,033,178 5/62 Koch 122-478 PERCY =L. PATRICK, Primary Examiner.

KENNETH W. SPRAGUE, Examiner. 

